Natural gas is often produced in sites remote from the places where it is intended to be used. A method used for transporting it consists in liquefying the natural gas around −160° C. and in transporting it by ship in liquid form at atmospheric pressure.
Prior to being liquefied, the natural gas is subjected to various treatments in order, on the one hand, to adjust its composition with a view to its sale (sulfur and carbon dioxide content, calorific value, etc.) and, on the other hand, to allow its liquefaction. The water has to be removed from the natural gas to prevent natural gas hydrates formation at low temperature, such hydrates being likely to clog the lines and the heat exchangers of the liquefaction plant. Benzene also crystallizes at low temperature and its proportion has to be adjusted below 1 ppm. The presence of heavy paraffins can also cause operating problems when they are in detectable proportions.
The usual treatments are a deacidizing stage in order to remove the hydrogen sulfide (H2S) and the carbon dioxide (CO2), followed by a dehydration stage. It is also common practice to collect the mercury contained in the natural gas, which would damage the devices made of aluminium alloy used to operate at cryogenic temperatures. During cooling of the natural gas, a fractionation stage during which the composition of the natural gas is adjusted by distillation in order to remove all the too heavy hydrocarbon compounds is commonly carried out. The proportion of ethane, propane and butane extracted by distillation is very large in relation to the heavier hydrocarbons, which are readily carried along to the bottom of the distillation column.
Problems arise when the natural gas is dry. A dry natural gas contains a very small proportion of hydrocarbons heavier than methane. Although the proportion of heavy hydrocarbons containing more than five carbon atoms is small, it is however necessary to remove these heavy hydrocarbons to prevent crystallization and clogging problems. Now, dry gases do not contain enough ethane, propane and butane to carry heavy hydrocarbons along towards the bottom of the distillation column. The distillation operation is therefore not efficient for removing the heavy hydrocarbons contained in a dry natural gas.
The present invention provides a liquefaction method comprising a stage of removing the water and the heavy compounds to allow liquefaction of a dry natural gas under good conditions.
The method according to the invention consists in adsorbing the water and the unwanted heavy hydrocarbons on at least two layers of different adsorbent materials prior to carrying out the high-pressure liquefaction stages.